How to Detect Fingerprints and Blood at a Crime Scene

There are many field tests available for testing the six forms of evidence: blood, narcotics, impressions, fingerprints, trace, and firearms and ammunition (CTU, 2006). Today, I want to focus on the reagent tests that are available for fingerprints and blood I have chosen these two types of reagent tests because they are the most common form and usually the most important pieces of evidence collected at a crime scene.

What is a reagent? A reagent, by definition, is "a chemical agent for use in chemical reactions" (Reagent, 2001). There are several forms of reagent tests that can be performed to develop fingerprints that are not visible to the naked eye. There are several reagents that are used to develop fingerprints at the crime scene. The following is a list and description of the various reagent tests available for fingerprints.

Ø Amido Black - dye that stains the protein found in blood and causes a blue-black reaction and should only be utilized to enhance blood-contaminated prints. This test should only be performed on non-porous materials. All blood samples should be obtained prior to utilizing Amido Black as it can destroy body fluid evidence (Warrington, 2005).

Ø Crazy glue (cyanaoacrylate) - easily lifts prints from surfaces such as cars, guns, and bullet casings. Should be used when brushing for prints would alter or destroy prints (Smith, 2006). Photographs should always be taken prior to using crazy glue (Smith).

Ø Ninhyndrin - is a chemical that bonds with the amino acids in fingerprints and will produce a blue or purple color (Smith). It is used to lift prints from surfaces such as paper and cardboard. Photographs should be taken to document the prints (Warrington).

Ø Powders - adhere to both water and fatty acids and are utilized on smoother surfaces such as tables and desks (Warrington). There are fluorescent powders that glow when a UV light is passed over the area, magnetic powders that use iron particles and a magnetic wand to produce and lift prints, and there are dusting powders that come in many colors to contrast with the background (O'Connor, 2004). All prints need to be photographed prior to lifting.

The detection of prints is very important in any homicide as it can determine who was present at the crime scene. The proper detection and collection of such prints can eliminate potential suspects, as well as place other suspects at the scene. Fingerprints can help law enforcement determine if the suspect acted alone or alongside others. Fingerprints on certain pieces of evidence may also determine if the victim(s) tried to fight back. Proper detection and documentation is vital when questioned during the court proceedings.

There are many blood reagent tests that are utilized to detect blood at the crime scene. There are characteristics that an investigator will want to have associated with any presumptive test for blood: sensitivity and specific to blood; reliable, stable, economical, and non-toxic; meet the Frye standard of general acceptance in the scientific community, does not destroy the evidence; and does not destroy the crime scene (Blood Intro, 2006). The reagent tests will produce either a color reaction or will glow depending on the test performed. The tests are based upon the properties of hemoglobin in the blood (Linville, 2006). There are color tests that provide a rapid change in color if the substance is positive for blood. There are five different color tests that can be performed: benzidine, phenolphthalein, o-tolidine, tetramethylbenzidine (TMB), and leucomalachite green (LMG) (Linville).

Benzidine will have a blue color if the substance is likely to be blood. This chemical is carcinogenic and is no longer in use by most scientists (Linville). Phenolphthalein is a chemical that is still utilized today and is usually referred to as the Kastle-Meyer test (Kastle-Meyer, 2006). When phenolphthalein reacts with the hemoglobin, it produces a pink color, but other substances may react with phenolphthalein producing a false positive (Linville). O-Tolidine is very similar in make as benzidine and produces a blue positive reaction. This reagent test has been gradually replaced by the use of tetramethylbenzidine because of its carcinogenic properties (Linville). Tetramethylbenzidine will produce a green to blue-green color when blood is the likely substance. The most common way to use this test is through Hemastix, which contains a plastic strip with a filter paper on the tip containing TMB and peroxidase (Linville). Luecomalachite Green will produce a positive result with a green color. This test is not nearly as accurate or sensitive as TMB and phenolphthalein (Linville).

The other reagent tests that are available for testing blood would be chemiluminescence, fluorescence, luminol, and fluorescein (Linville). These four reagent tests have a better accuracy rate and are more sensitive than the color tests. The downside of utilizing these reagent tests is that they may destroy valuable blood and DNA evidence (Linville). When these tests are utilized they produce a glowing color that identifies blood and can detect blood that is old and has been cleaned (Smith). These products should be used minimally such as spraying each corner and the middle of the room (Smith). This method of spraying will prevent too much damage being caused to DNA and blood evidence that may be present. When using these methods of collection, either videotape or photographs will need to be taken. The best method would be videotaping as it will produce a better visual of what happened at the scene (Smith).

The utilization of the blood reagent tests in a homicide case will provide an accurate documentation of the blood splatter and can detect fingerprints and smearing of blood. If there is unknown substances that look like blood, these reagent tests will help determine what the substance is. For instance, on a knife that is found at the scene, the investigators will be able to determine if the substance on the knife was actually blood or if it was some other substance. Just because an officer tells the investigator that there is blood at the scene, does not mean these tests should not be performed as the blood may be animal and not human (Smith).

Since photographs of both the fingerprints and the blood reagent tests will be taken there is specific information that should be noted in the case file. The following list should be found in the case file when documenting such evidence by photographs:

Ø Type of camera, brand, model, etc.

Ø Type of lens

Ø Type of film

Ø Shutter speed

Ø Lens opening

Ø The illumination

Ø Position of the camera

Ø Distance from the object being photographed or videotaped

Ø The angle of the photograph (O'Connor).

Also, there should be three different exposures taken: regular exposure, underexposure, and overexposure (O'Connor). This protects the police department from allegations of crime scene altering by providing a match to the documented information (O'Connor).

References

Blood Intro. (2006). An Introduction to Blood. Retrieved May 4, 2006 from

http://www.geocites.com/a4n6degener8/bloodintro.htm?20064 .

CTU. (2006). Task List. Retrieved May 4, 2006 from http://campus.ctuonline.edu .

Kastle-Meyer. (2006). How the Kastle-Meyer Test Works. Retrieved May 4, 2006 from

http://www.geocities.com/a4n6degener8/km_workings.htm?20064 .

Linville, J. (2006). Identification and Characterization of Blood and Bloodstains.

Retrieved May 4, 2006 from

http://www.dpo.uab.edu/~jglinvil/JS1105P05web/JS110,SP05,BloodID.ppt#156 .

O'Connor, T. (2004). Fingerprints and Trace Evidence. Retrieved May 4, 2006 from

http://faculty.ncwc.edu/toconnor/315/315lect05.htm .

Reagent. (2001). Definition of Reagent. Retrieved May 4, 2006 from

http://www.answers.com/main/ntquery?method=4&dsid=502&deid=45414253&curtab=502_1&linktext=Meaning%20%231 .

Smith, F. (2006). Final Chat Session - May 3, 2006. Retrieved May 3, 2006 from

https://campus.ctuonline.edu/MainFrame.aspx .